In vitro development of mouse embryos, as an experimental model system, is used to examine the activation of MHC genes during early mammalian development. Since MHC expression in the embryo dictates establishment of self tolerance and MHC restriction, we wish to determine the time, location and mechanism of onset of MHC expression. Blastocyst stage mouse embryos obtained from C3H strain mated with BALB/c males can be cultured in synthetic media containing fetal bovine serum and/or fresh rat serum. Almost all the embryos undergo in vitro differentiation to the neurula or to early somite stage. Only a fraction of embryos develop further to reach late somite stage in which regular movement of heart muscle can be observed. This system is superior to the study of in vivo developing embryos since it allows direct manipulation and excludes possible contamination by maternal tissues. MHC Class I antigen expression has been examined initially by the binding of rat monoclonal antibodies detecting the non-polymorphic part of antigens followed by studies with more specific mouse antibodies capable of distinguishing products of different regions. To evaluate the role of MHC antigen in the development of the immune system, the effect of monoclonal anti-H-2 antibodies on the developing mouse fetus is studied as a related, but independent approach. Pregnant females (starting 10 days after gestation) are being injected multiple times with large amounts of anti-H-2 antibodies with extremely high titer. These antibodies react with the H-2 antigens encoded by paternal genes but not with maternal H-2 antigens. We expect to observe modification of MHC antigen expression due to antibody exposure early in development and subsequent alteration in immune responsiveness in postnatal animals.